How to make weight paint mode use x-ray selecting
Yes, you can do that in weight paint mode:




Select the bone
Activate 'vertex selection masking' (or face selection masking with the button next to the left)
Now you can select vertices like in edit mode, using A or box selection or circle selection
Also you can use (if you need) Z to select behind/non visible parts (like toggling mesh/solid in edit mode, or like enabling/disabling 'limit selection to visible')